Composite artificial bone

ABSTRACT

An artificial bone which is easy to bond to a living bone and has a mechanical property approximate to that of a living bone is disclosed. The artificial bone comprises: a dense part made of titanium or a titanium alloy, in the shape of a frame that is approximate to a part of an outer face of a living bone, having a density of 95% or more; and a porous part made of sintered particles of titanium or a titanium alloy having the same or different composition as the titanium alloy for the dense part, in the shape approximate to the remaining part of the living bone, having a porosity of 40% or more, the dense part and the particles of the porous part being sintered to each other at an interface between the dense part and the porous part.

TECHNICAL FIELD

The present invention relates to an artificial bone made of titanium ora titanium alloy, in which a dense part and a porous part areintegrated.

BACKGROUND ART

Titanium is expected as a material for an artificial bone because it hashigher mechanical strength than ceramics and resins, and is able to havea bone forming ability only through an alkaline treatment. It is desiredthat a material for an artificial bone is a porous body having aporosity of 50% or higher, because of the necessity of forming a livingbone and bonding to a surrounding living bone after the material isimplanted into a living body. The mechanical strength that is requiredfor an artificial bone is 40 MPa or more for substitutes for spinealthough it depends on the site where it is implanted. It is known thata titanium porous body is obtained by sintering after mixing a titaniumpowder with a pore forming material as is necessary and pressure-moldingthe mixture (Patent document 1).

-   Patent document 1: JP2002-285203-A

DISCLOSURE OF THE INVENTION Problem to Be Solved by the Invention

However, when a porous body is used alone even if it is made of a metal,chips and breakages are likely to occur in a corner or an edge line, sothat it has poor reliability. In addition, since the modulus ofelasticity of a living bone largely differs depending on the site, as isevident from 10 to 20 GPa for a cortical bone, and 0.2 to 0.3 GPa for acancellous bone, it is difficult to adapt the modulus of elasticity ofan artificial bone to these values.

Therefore, it is an object of the present invention to provide anartificial bone which is easy to bond to a living bone and has amechanical property approximate to that of a living bone.

Means for Solving the Problem

In order to achieve the above object, the artificial bone of the presentinvention comprises

a dense part made of titanium or a titanium alloy, in the shape of aframe that is approximate to a part of an outer face of a living bone,having a density of 95% or more, and

a porous part made of sintered particles of titanium or a titaniumalloy, in the shape approximate to the remaining part of the livingbone, having a porosity of 40% or more.

The dense part and the particles of the porous part are sintered to eachother at an interface between the dense part and the porous part.

The titanium alloy for the porous part may have the same or differentcomposition as that for the dense part.

The part of an outer face of a living bone is typically a part of acortical bone, and the remaining part of the living bone is typicallythe remaining part of the cortical bone and a cancellous bone of theliving bone.

According to an experiment by the present inventor, it is necessary tomake the porosity 55% or less as shown in FIG. 1, for making a puretitanium porous body have a compressive strength of 40 MPa or more. Asshown in FIG. 2, the modulus of elasticity decreases with the porosityat porosities of up to 40%, while it gradually decreases at porositiesexceeding 40% and reaches about 2 GPa at a porosity of 70%.

On the other hand, since the artificial bone of the present inventionhas a dense part in the shape of a frame and a porous part having aporosity of 40% or more, biological tissues such as a living bone or abody fluid enter the porous part through an opening of the frame, andbond to the porous part. Since the dense part supports the load, thecompressive strength is high for the high porosity as a whole. Also, byappropriately defining the volume ratio between the porous part and thedense part, it is possible to adapt the modulus of elasticity to that ofa living bone. Further, when the part corresponding to a corner, an edgeline or a surface of a living bone is composed of the dense part, chipsand breakages can be prevented. Furthermore, when a sharp projection isformed on one end face of the dense part integrally, it is possible toachieve initial fixation by making this projection break into a livingbone.

Such an artificial bone is produced, for example, by calcining aftercombining a shaped body (a) of titanium or a titanium alloy powder notcontaining a pore forming agent (hereinafter, referred to as a “titaniumpowder and the like”) or a presintered body (A) thereof, and a shapedbody (b) of a mixture containing a titanium powder and the like and apore forming agent or a presintered body (B) thereof.

According to this production method, since the part derived from theshaped body (a) or the presintered body (A) does not contain a poreforming agent, it becomes a dense part after calcining. On the otherhand, since the part derived from the shaped body (b) or the presinteredbody (B) contains a pore forming agent, it becomes a porous part aftercalcining. In addition, since the shaped body (a) or the presinteredbody (A), and the shaped body (b) or the presintered body (B) arecombined and calcined together, particles inside each shaped body orparticles inside each presintered body are sintered together, and at thesame time, particles inside the shaped body (a) or the presintered body(A) and particles inside the shaped body (b) or the presintered body (B)are sintered at the interface. Therefore, the dense part and the porouspart are joined without intervened by any other matter than a transitionlayer therebetween.

As for the combination before calcining, both of them may be shapedbodies or presintered bodies, or either one of them may be a shaped bodyand the other of them may be a presintered body. Shaping into eachshaped body may be conducted before combining with the other shaped bodyor the like, or may be conducted simultaneously with combining. Anindividual shaping method for each shaped body may be known appropriatemeans including injection molding, pressure molding, extrusion molding,cast molding and sheet forming. In particular, it is preferred that theshaped body (b) is combined with the shaped body (a) simultaneously withthe shaping by placing the shaped body (a) in a die and charging theremaining space with the powder mixture, and then conducting pressuremolding. This allows combination with the shaped body (b) even when theshape of the shaped body (a) is complicated. When combining is madeafter conducting molding individually, it is preferred to conductfluid-pressure molding prior to calcining after combining.

Either one or both of the shaped body (a) and the shaped body (b) maycontain an organic binder, and when both of them contain organicbinders, the organic binders may be the same or different from eachother. When both of them contain binders, the pressure molding ispreferably conducted at a temperature at which the binders get soft.This is because particles in the interface will adhere to each othermore closely at the molding stage, and facilitate progress of sintering.

Unlike the aforementioned method, laser beam, electron beam, or thermalpoint source such as thin flame may be displaced to locally melt thetitanium powder, or different powders may be spot-welded to form asingle layer of predetermined shape, and a plurality of layers thusformed may be stacked to produce either one of the dense part and theporous part or to produce both at once.

As for the dense part, it may be produced by processing an expandedmaterial, without limited to the process including the steps of powder,shaping and sintering.

Effect of the Invention

Since the artificial bone of the present invention is easy to bond to aliving bone, and has a mechanical property that is approximate to thatof a living bone, the load on a patient who wears the same issignificantly reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relation between the porosity and thecompressive resistance of a titanium porous body.

FIG. 2 is a graph showing a relation between the porosity and themodulus of elasticity of a titanium porous body.

FIG. 3 is a drawing showing an initial step of a production method of anartificial bone according to Example 1.

FIG. 4 is a drawing showing an intermediate step of the same.

FIG. 5 is a CT image picture of a sintered body obtained in theintermediate step.

FIG. 6( a) is a drawing showing an initial step of an artificial boneaccording to Example 3, and FIG. 6( b) is an enlarged view of B part inFIG. 6( a).

FIG. 7 is drawings showing a production method of an artificial boneaccording to Example 4, in which FIG. 7( a) is a picture of a dense partalone, FIG. 7( b) is a picture of a porous part alone, and FIG. 7( c) isa picture of an artificial bone.

BEST MODES FOR CARRYING OUT THE INVENTION

An organic binder is composed, for example, of 0 to 50 vol % ofpolyacetal, 0 to 50 vol % of polypropylene and 50 to 70 vol % of waxes.The pressure at which pressure molding is conducted after placing ashaped body (a) in a die, and charging the remaining space with a powdermixture which is to be a shaped body (b) is preferably 20 to 100 MPa.When the pressure is less than 20 MPa, binding between a porous part anda dense part is insufficient, whereas when the pressure exceeds 100 MPa,the load exerted on the shaped body (a) is so large that the shaped body(a) may be broken. When shaping is conducted while the organic binder iscontained, it is preferred to remove the organic binder by heating inatmospheric air before calcining the combined body. When a shaped bodyin which the metal is titanium is presintered, heating is conducted at atemperature ranging from 800 to 1000° C. in a nonoxidative atmosphere.Final sintering is conducted at a temperature ranging from 1200 to 1400°C. in a nonoxidative atmosphere.

EXAMPLE 1

A pure titanium powder of Japanese Industrial Standard II (correspondingto US standard ASTM-G1) having a maximum particle size of 45 μm, and anorganic binder composed of 20 vol % of polyacetal, 20 vol % ofpolypropylene and 60 volt of waxes were mixed in a volume ratio of 65:35to prepare a mixture for dense part. The volume ratio was calculatedfrom the real density and the weight of each component. Then the mixturefor dense part was subjected to injection molding at 165° C. to obtain ashaped body (a1) having an outer dimension of 9.6 mm×12.0 mm×24.0 mm(width×depth×height) in which a square frame part formed of rightquadrangular prisms of 1.50 mm wide each is provided at its four cornerswith standing legs of right quadrangular prisms having the same width asshown in FIG. 3.

Separately, the pure titanium powder and ammonium hydrogen carbonatehaving a particle size adjusted to 500 to 1500 μm by means of a sievewere mixed in a volume ratio of 34:66, to obtain a mixture for porouspart. As shown in FIG. 4, the shaped body (a1) was placed in a diehaving an inner dimension slightly larger than the outer dimension ofthe shaped body (a1), and the remaining space was charged with themixture for porous part, and a pressure of 90 MPa was applied, to give acombined body in which a shaped body (b1) of an approximatelyrectangular parallelepiped was formed inside the shaped body (a1).

This combined body was calcined by retaining at 1250° C. for two hoursin an argon gas atmosphere. A CT image of the obtained sintered bodydemonstrated that the edge line part (dense part) and the inner part(porous part) were sintered while shapes originating from the shapedbodies (a1) and (b1) were respectively maintained as shown in FIG. 5.Also, particles in the interface were sintered in such a degree thatboundaries thereof were difficult to be recognized, and an artificialbone of 10 mm×8 mm×20 mm in which the dense part and the porous partwere integrated was obtained. A plurality of CT section images of thisartificial bone were taken, and the maximum diameter of certaindirection of pores was measured (maximum length of the gap part by aline of certain direction) on the obtained images. The pore diameter ofthe porous part fell within the range of 200 to 500 μm. This artificialbone was compressed in the axial direction at a compression speed of 1mm/min. and the compressive strength was found to be 53.3 MPa.Separately, the shaped bodies (a1) and (b1) were molded individuallywithout being combined with each other, and calcined in the samecondition as described above. The density and the porosity of theobtained sintered bodies were calculated from the weight and the realdensity of titanium. The density of the sintered body corresponding tothe dense part was 96%, and the porosity of the sintered bodycorresponding to the porous part was 60%. Based on values of 108 GPa and4.2 GPa that were obtained by individually measuring the modulus ofelasticity of the dense part and the porous part, the modulus ofelasticity of the entire sintered body was calculated. The result was15.8 GPa.

Next, the obtained artificial bone was immersed in a 5 M NaOH aqueoussolution at 60° C. for 24 hours, and sequentially immersed in pure waterat 40° C. for 12 hours, and then heated for an hour in an air furnace at600° C. The artificial bone was taken out of the furnace, and immersedin a simulated body fluid having an inorganic ion concentrationapproximately equal to that of a human body fluid (Japanese Patent No.2775523, column 6, lines 43 to 49), and deposition of an apatite phasewas observed in the dense part and the porous part after three days.

EXAMPLE 2

A combined body in which a shaped body (b2) of an approximatelyrectangular parallelepiped was molded inside the shaped body (a1) wasobtained in the same condition as that of Example 1 except that thevolume ratio between the titanium powder and ammonium hydrogen carbonatein the mixture for porous part was 43:57. This combined body wascalcined by retaining for two hours in an argon gas atmosphere at 1250°C. The density of the sintered body corresponding to the dense part was96%, and the porosity of the sintered body corresponding to the porouspart was 50.5%.

The obtained artificial bone was subjected to an alkali and heattreatment, and immersed in a simulated body fluid in a similar manner asin Example 1, and deposition of an apatite phase was observed in thedense part and in the porous part after three days.

EXAMPLE 3

The mixture for dense part formulated in Example 1 was injection-moldedat 165° C. to obtain a shaped body (a3) having an outer dimension of 30mm×20 mm×15 mm in which a plurality of right quadrangular pyramids of1.5 mm each on a side were arranged to stand up on one end face of arectangular parallelepiped frame part made of right quadrangular prismsof 3 mm wide each as shown in FIG. 6.

The shaped body (a3) was placed in a die having an inner dimension thatis slightly larger than the outer dimension of the shaped body (a3), andthe remaining space was charged with the mixture for porous partformulated in Example 1, and a pressure of 85 MPa was applied to give acombined body in which a shaped body (b3) of an approximatelyrectangular parallelepiped was molded inside the shaped body (a3).

This combined body was calcined in the same condition as in Example 1.The density of the sintered body corresponding to the dense part was97%, and the porosity of the sintered body corresponding to the porouspart was 61%. The compressive strength was 73 MPa, and the pore diameterof the porous part was within the range of 200 to 500 μm.

The obtained artificial bone was subjected to an alkali and heattreatment, and dipped in a pseudo body fluid in a similar manner as inExample 1, and deposition of an apatite phase was observed in the densepart and in the porous part after three days.

EXAMPLE 4

A shaped body (a4) having a soybean-like shape in a planar view as seenin a picture of FIG. 7( a), formed with a plurality of large pores on alateral face was obtained by processing a pure titanium expandedmaterial of Japanese Industrial Standard II (corresponding to USstandard ASTM-G1).

Separately, a mixture for porous part was prepared in the same conditionas in Example 1 and placed in a die, and a pressure of 90 MPa wasapplied, and heated for two hours in argon gas at 900° C. to give apresintered body (B4) having an outer shape which is complementary withan inner circumferential face of the shaped body (a4) except for theheight higher by 0.5 mm than the shaped body (a4) as shown in a pictureof FIG. 7( b).

After fitting the shaped body (a4) with the presintered body (B4), andapplying a pressure of 47 MPa in the height direction, heating wasconducted in argon gas at 1150° C. for two hours to produce anartificial bone as shown in FIG. 7( c).

The porosity of the porous part of the obtained artificial bone was 60%.Also it was found that the dense part and the porous part bindmicrostructually.

1-6. (canceled)
 7. A method of producing an artificial bone, the methodcomprising the steps of: preparing a die having an inner face that iscomplementary with a living bone; placing a shaped body (a) of titaniumor a titanium alloy powder not containing a pore forming agent, having aframe shape that is approximate with a part of an outer face of theliving bone, in the die ; charging the remaining space of the die with amixture containing titanium or a titanium alloy powder and a poreforming agent ; pressure-molding the mixture, to obtain a shaped body(b) and combine the shaped body (b) with the shaped body (a) ; andcalcining the combined body.
 8. The method according to claim 7, whereinthe part of the outer face of the living bone is a part including acorner, an edge line or a surface of the living bone.
 9. The methodaccording to claim 8, wherein the shaped body (a) has a sharp projectionon one end face.
 10. The method according to claim 7, wherein either oneor both of the shaped body (a) and the shaped body (b) contain anorganic binder.
 11. The method according to claim 7, wherein the organicbinder is composed of 0 to 50 vol % of polyacetal, 0 to 50 vol % ofpolypropylene and 50 to 70 vol % of waxes.
 12. The method according toclaim 7, wherein the pressure molding is conducted at a pressure of 20to 100 MPa.
 13. The method according to claim 7, wherein the poreforming agent contained in the mixture is ammonium hydrogen carbonate.14. The method according to claim 13, wherein the pore forming agentcontained in the mixture has a particle size adjusted to 500 to 1500 μmby means of a sieve.